Performance Analysis Of Reconfigurable Intelligent Surface Aided NOMA System

The future wireless network is expected to be a distributed intelligent wireless communication,sensing and computing platform,which connects the physical world and the digital world in a seamless and sustainable manner,and the key to realizing this challenging demand lies in the availability of a programmable wireless transmission environment,i.e.smart radio environment/intelligent radio environment(SRE/IRE).At present,a novel key technology for SRE is reconfigurable intelligent surfaces(RIS).On the other hand,non-orthogonal multiple access(NOMA)technology can perform multi-user multiplexing in the power domain,and the non-orthogonal resource allocation scheme also makes the number of users or devices that it can support no longer limited by the number of orthogonal resources,which improves the throughput and spectral efficiency of the system.Motivated by the advantages of both,this thesis combines RIS and NOMA technology,studies the RIS-aided NOMA system,and analyzes the impact of RIS on the performance of the NOMA system.Firstly,the robust beamforming design of the traditional passive RIS-aided NOMA system based on imperfect channel state information(CSI)is studied in this thesis.The error model of imperfect CSI in this thesis adopts two kinds of error models: quantization error model and statistical error model.In this thesis,non-convex quadratic expressions in optimization problems are transformed into a convex equivalent approximate expression by using S-Procedure and Bernstein inequality.At the same time,a method based on penalty function has been used to asymptotically obtain the rankone solution,and then an alternate optimization method is proposed to solve the coupled optimization variables.The simulation results verify the convergence of the alternate optimization algorithm,and show that with the increase of the number of RIS components,the performance gain of the system is significantly improved.And the simulation results also show the impact of error factors on system performance.Secondly,since traditional passive RIS cannot provide considerable gain for the system when the quality of direct channel is better or the fading of RIS reflection link is large,researchers put forward the concept of active RIS technology.Therefore,the beamforming design of active RIS-aided NOMA system has been further studied in this thesis,which analyzes the performance improvement of NOMA system by active RIS,and compares the performance differences between active RIS and passive RIS.The simulation results show that when the channel quality of the direct channel is good,the performance gain of passive RIS on the NOMA system is unnoticeable,while the active RIS can still provide considerable performance gain for the NOMA system.Finally,an eavesdropper is added to the active RIS-aided NOMA system to expand the scenario,and then the security communication problem of the active RIS-aided NOMA system has been studied in this thesis,where artificial noise is introduced into the system to further reduce the information leakage of legitimate users.In this thesis,the logarithm objective function is transformed by using the division property of logarithm,and the successive convex approximation method is adopted to reduce the complexity of solving the optimization problem.The simulation results show that the gain of active RIS on the secure communication rate of the NOMA system is higher than that of passive RIS when the fading of RIS reflection link is large,and the artificial noise also plays an active role in suppressing information leakage.At the same time,the simulation results also show that the amplitude parameter in the reflection coefficient of active RIS is a key optimization parameter,and the active RIS can provide higher performance gains than the passive RIS in the scenario of a small deployment area.The above research verifies that by optimizing the beamforming matrix at the BS and the reflection coefficient of the active RIS respectively,RIS can effectively improve the energy efficiency and spectrum efficiency of the NOMA system,and effectively suppress the information leakage of legitimate users.